As an alternative for the well known halogen based-flame retardant containing systems, in the last years the market is increasingly interested in halogen free flame retardant additives to be used in the production of corresponding flame-retardant compositions, generally speaking thermoplastic compositions.
Significant requirements for these products are: high flame retardant effectiveness in reinforced and unreinforced articles, pale intrinsic colour, good heat stability and good mechanical and electrical properties.
The known halogen-free flame retardant additives mostly used in thermoplastic polymers are:                Inorganic flame retardants like Magnesium hydroxide. The product must be used in large amounts to be effective so that the mechanical properties of the related thermoplastic articles are dramatically damaged.        Melamine derivatives, like melamine cyanurate, melamine (poly)phosphate or melamine pyrophosphate. These products do not have sufficient thermal stability for overcoming the processing conditions of glass fibre reinforced polyamides and therefore cannot be used as universal additives.        Organic derivatives of phosphoric acid such as phosphoric esters (valence state of P=+5). These products are not very effective, tend to bleed out on exposure to heat, are generally in a liquid status, being therefore difficult to handle, are not hydrolytically stable, therefore affect mechanical and thermal properties of final articles. Although many improvements have been introduced with the new high molecular weight products, however the related polymer articles do not have fully satisfactory performances due to the unbalanced combination of flammability with impact resistance, heat stability and weather resistance.        Red-Phosphorus is the most effective flame retardant additive especially for glass fiber reinforced polyamides but unfortunately its inherent deep-red colour makes the related polymer product difficult to be used for natural or light colours applications.        Organic phosphinates with particular regard to aluminium and zinc phosphinates, have been also described as flame retardant additives for reinforced polyamides (U.S. Pat. No. 6,547,992; U.S. Pat. No. 6,255,371) especially in combination with nitrogen containing compounds like melamine derivatives, tris(hydroxyethyl)isocyanurate, benzoguanamine, allantoin or similar products (EP 97/01664).        Hypophosphorous acid metal salts are also known with the chemical name of inorganic phosphinates. These products have been recently described (U.S. Pat. No. 4,618,633) as halogen free flame retardant agents for fire proofing styrene containing polymers like: polystyrene (PS), high impact polystyrene (HIPS), styrene-acrylonitrile copolymers (SAN), acrylonitrile-butadiene-styrene polymers (ABS). More recently, Calcium hypophosphite has been reported to be especially effective in polyester resin compositions when used in combination with nitrogen containing compounds like: melamine cyanurate, melamine polyphosphate, melamine borate, melamine pyrophosphate, ureaphosphate or similar products (U.S. Pat. No. 6,503,969; WO 09817720; DE 19904814; DE 10137930; EP 0919591) but it does not work in polyamide articles.        